Currently, as bonding wires for semiconductor devices (hereinafter referred to as “bonding wires”) for bonding electrodes on semiconductor elements with external leads, fine wires approximately 15 to 50 μm in wire diameter are mainly used. As a method for bonding the bonding wire, a thermosonic bonding technique is used commonly, and a capillary is used on a general-purpose bonding machine, as a jig used to connect a bonding wire passed therethrough. A bonding wire bonding process involves heat-melting a wire tip by arc heat input, forming a ball by surface tension, followed by crimp-bonding the ball portion (hereinafter referred to as “ball bonding”) onto an electrode of a semiconductor element heated in a range of 150 to 300° C., then forming a loop, and crimp-bonding a wire portion to an electrode on the side of the external lead (hereinafter referred to as “wedge bonding”) to complete the process. An electrode structure in which an alloy film composed principally of Al is formed on a Si substrate is mainly used for the electrode on the semiconductor element to which the bonding wire is bonded while an electrode structure plated with Ag or Pd is mainly used for the electrode on the side of the external lead.
Au has mainly been used as a bonding wire material because of excellent oxidation resistance and good bondability of Au. However, due to recent escalation of Au prices, there has been demand for development of a less expensive, and a highly functional bonding wire. In response to this demand, a bonding wire made of Cu is proposed by taking advantage of low cost and high electric conductivity. Regarding the bonding wire made of Cu, bonding wires made of high-purity Cu (with a purity of 99.99 wt. % or above) are proposed (e.g., Patent Literature 1). The Cu bonding wire made of high-purity Cu offers the advantage of having excellent electric conductivity and other advantages, but has problems in that bondability is deteriorated by surface oxidation, that the life of ball bonds (hereinafter referred to as “bonding reliability”) in high-temperature and high-humidity environments is shorter than an Au bonding wire, and so on. As a technique for solving the above problems, a bonding wire produced by coating a surface of a Cu alloy core material with Pd (hereinafter referred to as a “Pd-coated Cu bonding wire”) has been proposed (e.g., Patent Literature 2 and Patent Literature 3). By coating the Cu alloy core material with Pd having excellent oxidation resistance, the Pd-coated Cu bonding wire features excellent bondability and improved bonding reliability in high-temperature and high-humidity environments as well.